Liquid crystal display device

ABSTRACT

A liquid crystal display device comprises: a first display panel displaying a color image; a second display panel displaying a monochrome image; and an image processor generating first image data corresponding to the color image and second image data corresponding to the monochrome image based on an input video signal. The image processor generates the first image data and the second image data such that a graph representing transmittance of the first display panel for input gradation corresponding to the input video signal and a graph representing transmittance of the second display panel for the input gradation corresponding to the input video signal intersect each other at predetermined input gradation.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from Japanese application JP2016-229154, filed Nov. 25, 2016. This Japanese application isincorporated herein by reference.

TECHNICAL FIELD

A present invention relates to a liquid crystal display device.

BACKGROUND

A technique, in which two display panels overlap each other and an imageis displayed on each display panel based on an input video signal, isconventionally proposed to improve contrast of a liquid crystal displaydevice (for example, see Unexamined Japanese Patent Publication No.2007-310161). Specifically, for example, a color image is displayed on afront-side (observer side) display panel in two display panels disposedback and forth, and a monochrome image is displayed on a rear-side(backlight side) display panel. Thus, contrast is improved. In theliquid crystal display device, adjustment is performed such that a wholegamma value of both the display panels becomes 2.2.

SUMMARY

In the conventional liquid crystal display device, when the image isdisplayed in low gradation, black floating occurs and thus displayquality is degraded.

The present disclosure is devised from a viewpoint of the abovecircumstance, and its object is to suppress the degradation of thedisplay quality during the display of the low-gradation image in theliquid crystal display device configured by overlapping the plurality ofdisplay panels each other.

To solve the above problem, a liquid crystal display device according tothe present disclosure, in which a plurality of display panels overlapeach other and an image is displayed on each of the display panels, theliquid crystal display device comprises: a first display panel thatdisplays a color image; a second display panel that displays amonochrome image; and an image processor that generates first image datacorresponding to the color image and second image data corresponding tothe monochrome image based on an input video signal, wherein the imageprocessor generates the first image data and the second image data suchthat a graph representing transmittance of the first display panel forinput gradation corresponding to the input video signal and a graphrepresenting transmittance of the second display panel for the inputgradation corresponding to the input video signal intersect each otherat predetermined input gradation.

In the liquid crystal display device according to the presentdisclosure, the graph representing the transmittance of the firstdisplay panel may have a first inflection point and changes to anupwardly convex state in a region lower than or equal to thepredetermined input gradation, and the graph representing thetransmittance of the second display panel may have a second inflectionpoint and changes to a downwardly convex state in the region lower thanor equal to the predetermined input gradation.

In the liquid crystal display device according to the presentdisclosure, the graph representing the transmittance of the firstdisplay panel may have a third inflection point and changes to adownwardly convex state in a region higher than the predetermined inputgradation, and the graph representing the transmittance of the seconddisplay panel may have a fourth inflection point and changes to aupwardly convex state in the region higher than the predetermined inputgradation.

In the liquid crystal display device according to the presentdisclosure, the transmittance of the first display panel may be largerthan the transmittance of the second display panel in a region lowerthan or equal to the predetermined input gradation, and thetransmittance of the second display panel may be larger than thetransmittance of the first display panel in a region higher than thepredetermined input gradation.

In the liquid crystal display device according to the presentdisclosure, the image processor may include a first image data generatorthat decides first gradation corresponding to the first image data and asecond image data generator that decides second gradation correspondingto the second image data, the second image data generator may decide thesecond gradation lower than the first gradation based on the inputgradation corresponding to the input video signal in a region lower thanor equal to the predetermined input gradation, and the first image datagenerator may decide the first gradation based on the second gradationdecided by the second image data generator.

The liquid crystal display device according to the present disclosuremay further comprises a correction table in which the second gradationis correlated with a correction value to be used for correcting theinput gradation, wherein the first image data generator may refer to thecorrection table to obtain the correction value, which is correlatedwith the second gradation decided by the second image data generator,and may correct the input gradation to calculate the first gradationbased on the obtained correction value.

In the liquid crystal display device according to the presentdisclosure, the correction value may be set such that a compoundtransmittance in which the transmittance of the first display panel andthe transmittance of the second display panel are compounded comes closeto transmittance corresponding to a predetermined gamma value.

A liquid crystal display device according to an another presentdisclosure, in which a plurality of display panels overlap each otherand an image is displayed on each of the display panels, the liquidcrystal display device comprises: a first display panel that displays acolor image; a second display panel that displays a monochrome image;and an image processor that generates first image data corresponding tothe color image and second image data corresponding to the monochromeimage based on an input video signal, wherein the image processorgenerates the first image data such that a graph representingtransmittance of the first display panel for input gradationcorresponding to the input video signal has a first inflection point andchanges to an upwardly convex state in a region lower than or equal topredetermined input gradation.

In the liquid crystal display device according to the another presentdisclosure, the graph representing the transmittance of the firstdisplay panel may have a second inflection point and may change to adownwardly convex state in a region higher than the predetermined inputgradation.

In the liquid crystal display device according to the another presentdisclosure, the image processor may generate the second image data suchthat a graph representing transmittance of the second display panel forinput gradation corresponding to the input video signal has a thirdinflection point and changes to a downwardly convex state in the regionlower than or equal to the predetermined input gradation.

In the liquid crystal display device according to the another presentdisclosure, the graph representing the transmittance of the seconddisplay panel may have a fourth inflection point and may change to anupwardly convex state in the region higher than the predetermined inputgradation.

A liquid crystal display device according to another present disclosure,in which a plurality of display panels overlap each other and an imageis displayed on each of the display panels, the liquid crystal displaydevice comprises: a first display panel that displays a color image; asecond display panel that displays a monochrome image; and an imageprocessor that generates second image data corresponding to the colorimage and second image data corresponding to the monochrome image basedon an input video signal, wherein the image processor generates thesecond image data such that a graph representing transmittance of thesecond display panel for input gradation corresponding to the inputvideo signal has a first inflection point and changes to a downwardconvex state in a region lower than or equal to predetermined inputgradation.

In the liquid crystal display device according to the another presentdisclosure, the graph representing the transmittance of the seconddisplay panel may have a second inflection point and may change to anupward convex state in a region higher than the predetermined inputgradation.

According to the present disclosure, it is possible to suppress thedegradation of the display quality during the display of thelow-gradation image in the liquid crystal display device configured byoverlapping the plurality of display panels each other.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view illustrating a schematic configuration of a liquidcrystal display device according to the present exemplary embodiment;

FIG. 2 is a plan view illustrating a schematic configuration of a firstdisplay panel according to the present exemplary embodiment;

FIG. 3 is a plan view illustrating a schematic configuration of a seconddisplay panel according to the present exemplary embodiment;

FIG. 4 is a sectional view taken along a line A-A′ in FIGS. 2 and 3;

FIG. 5A is a plan view illustrating another example of pixeldispositions of the first display panels;

FIG. 5B is a plan view illustrating another example of pixeldispositions of the second display panels;

FIG. 6 is a block diagram illustrating a specific configuration of animage processor according to an exemplary embodiment;

FIG. 7A is a graph representing a relationship between an inputgradation and first and second gradations;

FIG. 7B is an enlarged view of a low-gradation region in FIG. 7A;

FIG. 8 is a graph representing a relationship between an input gradationand a correction gradation;

FIG. 9A is a graph representing a relationship between an inputgradation and first and second transmittances;

FIG. 9B is an enlarged view of a low-gradation region in FIG. 9A;

FIG. 10 is a graph representing a relationship between an inputgradation and first, second and correction transmittances;

FIG. 11 is a graph representing a compound transmittance composed by atransmittance of a first display panel in FIG. 10 and a transmittance ofa second display panel;

FIG. 12 is a graph representing a comparative example of the graph inFIG. 11; and

FIG. 13 is a graph representing transmittance of the first displaypanel, transmittance of the second display panel and gamma transmittancecorresponding to the input gradation in the liquid crystal displaydevice of the comparative example.

DETAILED DESCRIPTION

Hereinafter, exemplary embodiments of the present disclosure will bedescribed with reference to the drawings. A liquid crystal displaydevice according to the present exemplary embodiment includes aplurality of display panels that display images, a plurality of drivingcircuits (a plurality of source drivers and a plurality of gate drivers)that drive the display panels, a plurality of timing controllers thatcontrol the driving circuits, an image processor that performs imageprocessing on an input video signal input from an outside and outputsimage data to each of the timing controllers, and a backlight thatirradiates the plurality of display panels with light from a rearsurface side. There is no limitation to a number of display panels, butit is only necessary to provide at least two display panels. When viewedfrom an observer side, the plurality of display panels are disposedwhile overlapping each other in a front-back direction. An image isdisplayed on each of the display panels. Liquid crystal display device10 including two display panels will be described below by way ofexample.

FIG. 1 is a plan view illustrating a schematic configuration of liquidcrystal display device 10 according to the present exemplary embodiment.As illustrated in FIG. 1, liquid crystal display device 10 includesfirst display panel 100 disposed closer to an observer (front side),second display panel 200 disposed farther away from the observer (rearside) than first display panel 100, first timing controller 140 thatcontrols first source drivers 120 and first gate drivers 130, firstsource drivers 120 and first gate drivers 130 being provided in firstdisplay panel 100, second timing controller 240 that controls secondsource drivers 220 and second gate drivers 230, second source drivers220 and second gate drivers 230 being provided in second display panel200, and image processor 300 that outputs image data to first timingcontroller 140 and second timing controller 240. First display panel 100displays a color image in first image display region 110 according tothe input video signal, and second display panel 200 displays amonochrome image in second image display region 210 according to theinput video signal. Image processor 300 receives input video signal Datatransmitted from an external system (not illustrated), performs imageprocessing (to be described later) on input video signal Data, outputsfirst image data DAT1 to first timing controller 140, and outputs secondimage data DAT2 to second timing controller 240. Image processor 300also outputs a control signal (not illustrated in FIG. 1) such as asynchronizing signal to first timing controller 140 and second timingcontroller 240. First image data DAT1 is image data for displaying thecolor image, and second image data DAT2 is image data for displaying themonochrome image. It is also possible that first display panel 100 maydisplays a monochrome image in first image display region 110 and seconddisplay panel 200 may display a monochrome image in second image displayregion 210. A backlight (not illustrated in FIG. 1) is disposed on arear surface side of second display panel 200. A specific configurationof image processor 300 will be described later.

FIG. 2 is a plan view illustrating a schematic configuration of firstdisplay panel 100, and FIG. 3 is a plan view illustrating a schematicconfiguration of second display panel 200. FIG. 4 is a sectional viewtaken along a line A-A′ in FIGS. 2 and 3.

A configuration of first display panel 100 will be described withreference to FIGS. 2 and 4. As illustrated in FIG. 4, first displaypanel 100 includes thin film transistor substrate 101 (hereinafter, TFTsubstrate) disposed on the side of backlight 400, opposed substrate 102,which is disposed on the observer side while being opposite to TFTsubstrate 101, and liquid crystal layer 103 disposed between TFTsubstrate 101 and opposed substrate 102. Polarizing plate 104 isdisposed on the side of backlight 400 of first display panel 100, andpolarizing plate 105 is disposed on the observer side.

In TFT substrate 101, as illustrated in FIG. 2, a plurality of datalines 111 (source line) extending in a first direction (for example, acolumn direction), a plurality of gate lines 112 extending in a seconddirection (for example, a row direction) different from the firstdirection are formed, and thin film transistor 113 (hereinafter, TFT) isformed near an intersection between corresponding one of data lines 111and corresponding one of gate lines 112. In planar view of first displaypanel 100, a region surrounded by two data lines 111 adjacent to eachother and two gate lines 112 adjacent to each other is defined as onesub-pixel 114, and a plurality of sub-pixels 114 are arranged in amatrix form (in the row and column directions). The plurality of datalines 111 are disposed at equal intervals in the row direction, and theplurality of gate lines 112 are disposed at equal intervals in thecolumn direction. In TFT substrate 101, pixel electrode 115 is formed ineach sub-pixel 114, and one common electrode (not illustrated) common tothe plurality of sub-pixels 114 is formed. A drain electrodeconstituting TFT 113 is electrically connected to data line 111, asource electrode constituting TFT 113 is electrically connected to pixelelectrode 115, and a gate electrode constituting TFT 113 is electricallyconnected to gate line 112.

As illustrated in FIG. 4, a plurality of color filters 102 a (coloredportions) each of which corresponds to sub-pixel 114 are formed onopposed substrate 102. Each color filter 102 a is surrounded by blackmatrix 102 b blocking light transmission. For example, each color filter102 a is formed into a rectangular shape. The plurality of color filters102 a include red color filters made of a red (R color) material totransmit red light, green color filters made of a green (G color)material to transmit green light, and blue color filters made of a blue(B color) material to transmit blue light. One of the red color filters,one of the green color filters, and one of the blue color filters arerepeatedly arranged in this order in the row direction, color filtershaving the same color are arranged in the column direction, and blackmatrices 102 b are formed in boundaries of color filters 102 a adjacentin the row and column directions. In accordance with color filters 102a, the plurality of sub-pixels 114 include red sub-pixels 114Rcorresponding to the color filters, green sub-pixels 114G correspondingto the green color filters, and blue sub-pixels 114B corresponding tothe blue color filters as illustrated in FIG. 2. In first display panel100, one pixel 124 is constructed with one red sub-pixel 114R, one greensub-pixel 114G, and one blue sub-pixel 114B, and a plurality of pixels124 are arranged in a matrix form.

First timing controller 140 has a known configuration. For example,based on first image data DAT1 and first control signal CS1 (such as aclock signal, a vertical synchronizing signal, and a horizontalsynchronizing signal), which are output from image processor 300, firsttiming controller 140 generates various timing signals (data start pulseDSP1, data clock DCK1, gate start pulse GSP1, and gate clock GCK1) tocontrol first image data DA1 and drive of first source driver 120 andfirst gate driver 130 (see FIG. 2). First timing controller 140 outputsfirst image data DA1, data start pulse DSP1, and data clock DCK1 tofirst source driver 120, and outputs gate start pulse GSP1 and gateclock GCK1 to first gate driver 130.

First source driver 120 outputs a data signal (data voltage)corresponding to first image data DA1 to data lines 111 based on datastart pulse DSP1 and data clock DCK1. First gate driver 130 outputs agate signal (gate voltage) to gate lines 112 based on gate start pulseGSP1 and gate clock GCK1.

The data voltage is supplied from first source driver 120 to each dataline 111, and the gate voltage is supplied from first gate driver 130 toeach gate line 112. Common voltage V_(com) is supplied from a commondriver (not illustrated) to the common electrode. When the gate voltage(gate-on voltage) is supplied to gate line 112, TFT 113 connected togate line 112 is turned on, and the data voltage is supplied to pixelelectrode 115 through data line 111 connected to said TFT 113. Anelectric field is generated by a difference between the data voltagesupplied to pixel electrode 115 and common voltage V_(com) supplied tothe common electrode. The liquid crystal is driven by the electricfield, and transmittance of backlight 400 is controlled, therebydisplaying an image. In first display panel 100, the color image isdisplayed by supply of a desired data voltage to data line 111 connectedto pixel electrode 115 of each of red sub-pixel 114R, green sub-pixel114G, and blue sub-pixel 114B. A known configuration can be applied tofirst display panel 100.

Next, a configuration of second display panel 200 will be describedbelow with reference to FIGS. 3 and 4. As illustrated in FIG. 4, seconddisplay panel 200 includes TFT substrate 201 disposed on the side ofbacklight 400, opposed substrate 202, which is disposed on the observerside while being opposite to TFT substrate 201, and liquid crystal layer203 disposed between TFT substrate 201 and opposed substrate 202.Polarizing plate 204 is disposed on the side of backlight 400 of seconddisplay panel 200, and polarizing plate 205 is disposed on the observerside. Diffusion sheet 301 and/or adhesive sheet are disposed betweenpolarizing plate 104 of first display panel 100 and polarizing plate 205of second display panel 200.

In TFT substrate 201, as illustrated in FIG. 3, a plurality of datalines 211 (source lines) extending in the column direction, a pluralityof gate lines 212 extending in the row direction are formed, and TFT 213is formed near an intersection between corresponding one of data lines211 and corresponding one of gate lines 212. In planar view of seconddisplay panel 200, a region surrounded by two data lines 211 adjacent toeach other and two gate lines 212 adjacent to each other is defined asone pixel 214, and a plurality of pixels 214 are arranged in a matrixform (the row direction and the column direction). The plurality of datalines 211 are disposed at equal intervals in the row direction, and theplurality of gate lines 212 are disposed at equal intervals in thecolumn direction. In TFT substrate 201, pixel electrode 215 is formed ineach pixel 214, and one common electrode (not illustrated) common to theplurality of pixels 214 is formed. A drain electrode constituting TFT213 is electrically connected to data line 211, a source electrodeconstituting TFT 213 is electrically connected to pixel electrode 215,and a gate electrode constituting TFT 213 is electrically connected togate line 212. Sub-pixel 114 of first display panel 100 and pixel ofsecond display panel 200 are disposed on one-to-one correspondence, andoverlap each other in planar view. For example, red sub-pixel 114R,green sub-pixel 114G and blue sub-pixel 114B, which constitute pixel 124in FIG. 2, and three pixels 214 in FIG. 3 overlap each other in planarview. It is also possible that a relationship between sub-pixels 114 offirst display panel 100 and pixels 214 of second display panel 200 isthree to one. As illustrated in FIGS. 5A and 5B, one pixel 124 (see FIG.5A) composed of red sub-pixel 114R, green sub-pixel 114G and bluesub-pixel 114B of first display panel 100 may overlap one pixel 214 (seeFIG. 5B) of second display panel 200 in planar view.

As illustrated in FIG. 4, in opposed substrate 202, black matrix 202 bblocking light transmission is formed at a position corresponding to aboundary of each pixel 214. The color filter is not formed in region 202a surrounded by black matrix 202 b. For example, an overcoat film isformed in region 202 a.

Second timing controller 240 has a known configuration. For example,based on second image data DAT2 and second control signal CS2 (such as aclock signal, a vertical synchronizing signal, and a horizontalsynchronizing signal), which are output from image processor 300, secondtiming controller 240 generates various timing signals (data start pulseDSP2, data clock DCK2, gate start pulse GSP2, and gate clock GCK2) tocontrol second image data DA2 and drive of second source driver 220 andsecond gate driver 230 (see FIG. 3). Second timing controller 240outputs second image data DA2, data start pulse DSP2, and data clockDCK2 to second source driver 220, and outputs gate start pulse GSP2 andgate clock GCK2 to second gate driver 230.

Second source driver 220 outputs the data voltage corresponding tosecond image data DA2 to data lines 211 based on data start pulse DSP2and data clock DCK2. Second gate driver 230 outputs the gate voltage togate lines 212 based on gate start pulse GSP2 and gate clock GCK2.

The data voltage is supplied from second source driver 220 to each dataline 211, and the gate voltage is supplied from second gate driver 230to each gate line 212. Common voltage V_(com) is supplied from thecommon driver to the common electrode. When the gate voltage (gate-onvoltage) is supplied to gate line 212, TFT 213 connected to gate line212 is turned on, and the data voltage is supplied to pixel electrode215 through data line 211 connected to said TFT 213. An electric fieldis generated by a difference between the data voltage supplied to pixelelectrode 215 and common voltage V_(com) supplied to the commonelectrode. The liquid crystal is driven by the electric field, andtransmittance of backlight 400 is controlled, thereby displaying animage. The monochrome image is displayed on second display panel 200. Aknown configuration can be applied to second display panel 200.

FIG. 6 is a block diagram illustrating a specific configuration of imageprocessor 300. Image processor 300 includes first delay unit 311, firstimage data generator 312, correction table 313, first image output unit314, second image data generator 321, gradation table 322, extensionfilter processor 323, average value filter processor 324, second delayunit 325, and second image output unit 326. Image processor 300 performsimage processing (to be described later) based on input video signalData, and generates first image data DAT1 (color image data) for firstdisplay panel 100 and second image data DAT2 (monochrome image data) forsecond display panel 200. Image processor 300 generates first image dataDAT1 and second image data DAT2 such that a compound gamma value (γvalue) of the display image in which the color image and the monochromeimage are compounded becomes a desired value (for example, γ=2.2).

When receiving input video signal Data transmitted from an externalsystem, image processor 300 transfers input video signal Data to firstdelay unit 311 and second image data generator 321. Input video signalData includes luminance information (gradation information) and colorinformation. The color information is information designating color. Forexample, each of a plurality of colors including an R color, a G color,and a B color can be expressed by 0 to 1023 values in a case where inputvideo signal Data is 10 bits. Each of the plurality of colors includingthe R color, the G color, and the B color can be expressed by 0 to 255values in a case where input video signal Data is 8 bits. The pluralityof colors includes at least the R color, the G color, and the B color,and may further include a W (white) color and/or a Y (yellow) color. Thecase where input video signal Data is 10 bits while the plurality ofcolors includes the R color, the G color, and the B color will bedescribed below as an example. In the case where input video signal Datais 10 bits, the gradation is expressed by 0 to 1023 steps.

When obtaining input video signal Data, second image data generator 321refers to gradation table 322 (gradation LUT) to decide the gradation(second gradation) corresponding to the monochrome image data. Graph (1)in FIGS. 7A and 7B represents an example of gradation table 322. FIG. 7Bis an enlarged view of a low-gradation region in FIG. 7A. In gradationtable 322 (graph (1) in FIGS. 7A and 7B), the gradation (inputgradation) corresponding to input video signal Data is correlated withthe gradation (second gradation) corresponding to the monochrome imagedata. For example, in a case where the input gradation is “2-stepgradation”, second image data generator 321 decides “26-step gradation”correlated with “2-step gradation” as the second gradation.

When obtaining the monochrome image data (second gradation) from secondimage data generator 321, extension filter processor 323 performsextension filtering for extending a high-luminance region on themonochrome image data. Specifically, extension filter processor 323performs processing (extension filtering) for setting the luminance ofeach pixel (target pixel) to the maximum luminance in a predeterminedfilter size. The high-luminance regions (for example, a white region)extend as a whole through the extension filtering. The filter size isnot limited to the 11-by-11 pixel region. The filter shape is notlimited to the square shape, but the filter may be formed into acircular shape.

When obtaining the monochrome image data that has been subject to theextension filtering, average value filter processor 324 performssmoothing on the monochrome image data using an average value filtercommon to all pixels 214 in each frame. For example, using the 11-by-11pixel region configured by each 11 pixels on the right, left, top, andbottom around each pixel 214 (target pixel) as the filter size, averagevalue filter processor 324 performs processing for setting a luminanceof target pixel 214 to the average luminance in the filter size.Although the filter size is not limited to the 11-by-11 pixel region,all pixels 214 are set to the common filter size in each frame. Thefilter is not limited to the square shape, but the filter may be formedinto a circular shape. A high-frequency component is deleted through thesmoothing, so that a luminance change can be smoothed. Average valuefilter processor 324 outputs the monochrome image data that has beensubject to the smoothing to first image data generator 312 and seconddelay unit 325.

Based on the monochrome image data (second gradation) obtained fromaverage value filter processor 324 and input video signal Data obtainedfrom first delay unit 311, first image data generator 312 decides thegradation (first gradation) of the color image displayed on firstdisplay panel 100. Specifically, when obtaining the second gradation,first image data generator 312 transforms the second gradation intopre-correction gradation using the following equation (1).Pre-correction gradation=1023×input gradation/second gradation  (1)

For example, in a case where the input gradation is “2-step gradation”while the second gradation is “26-step gradation” (graph (1) in FIGS. 7Aand 7B), the pre-correction gradation becomes “79-step gradation” inaccordance with the equation (1). Graph (2) in FIGS. 7A and 7Brepresents the pre-correction gradation calculated using the equation(1) in a case where gradation table 322 is set to graph (1) in FIGS. 7Aand 7B.

Then, first image data generator 312 refers to correction table 313(correction LUT) to obtain a correction value to be used for correctingthe pre-correction gradation in order to calculate the first gradation.Graph (4) in FIG. 8 illustrates an example of correction table 313. Incorrection table 313 (graph (4) in FIG. 8), the second gradation and thecorrection value are correlated with each other. For example, in a casewhere the second gradation is “26-step gradation”, first image datagenerator 312 obtains “9.5” correlated with “26-step gradation” as thecorrection value. The correction value (graph (4)) of correction table313 is set according to an inverse number (graph (5)) of the set gammavalue (2.2) in a region higher than the low-gradation region of 0-stepgradation to t3-step gradation, and is set to a value for correcting thepre-correction gradation to a lower level in the low-gradation region of0-step gradation to t3-step gradation.

Then, first image data generator 312 decides the first gradation(post-correction gradation) based on the obtained correction value(gradation correction processing). Specifically, first image datagenerator 312 decides a value obtained by multiplying the inputgradation by the correction value as the first gradation. For example,in a case where the input gradation is “2-step gradation” while thecorrection value is “9.5” (graph (4) in FIG. 8), the first gradationbecomes “19-step gradation” (=2-step gradation×9.5-step gradation).Graph (3) in FIGS. 7A and 7B represents the first gradation, which iscalculated in a case where gradation table 322 is set to graph (1) inFIGS. 7A and 7B while correction table 313 is set to graph (4) in FIG.8. As illustrated in FIGS. 7A and 7B, the first gradation (graph (3)) islower than the pre-correction gradation (graph (2)) in a region lowerthan or equal to predetermined gradation (t1-step gradation), and isequal to the pre-correction gradation in a region higher than thepredetermined gradation (t1-step gradation). First image data generator312 outputs the generated color image data (first gradation) to firstimage output unit 314.

Second delay unit 325 outputs the monochrome image data (secondgradation) that has been subject to the smoothing to second image outputunit 326 in synchronization with timing of outputting the color imagedata (first gradation) in first image data generator 312.

First image output unit 314 outputs the color image data (firstgradation) to first timing controller 140 as first image data DAT1.Second image output unit 326 outputs the monochrome image data (secondgradation) to second timing controller 240 as second image data DAT2.Image processor 300 outputs first control signal CS1 to first timingcontroller 140, and outputs second control signal CS2 to second timingcontroller 240 (see FIGS. 2 and 3).

As described above, in liquid crystal display device 10 according to thepresent exemplary embodiment, image processor 300 sets the firstgradation and the second gradation such that the first gradation (graph(3)) is higher than the second gradation (graph (1)) in the region (thelow-gradation region of 0-step gradation to t2-step gradation) lowerthan or equal to the predetermined gradation (t2-step gradation), andsuch that the second gradation (graph (1)) is higher than the firstgradation (graph (2)) in the region higher than the predeterminedgradation (t2-step gradation) as illustrated in FIGS. 7A and 7B. Whenattention is paid to the gradation characteristic in FIGS. 7A and 7B,graph (3) representing the gradation characteristic corresponding tofirst image data DAT1 (first gradation) for first display panel 100 hasan inflection point and changes to a convex state in the region lowerthan or equal to the predetermined input gradation (t2-step gradation).

FIGS. 9A and 9B are graphs representing first transmittance (graph (6))of first display panel 100, second transmittance (graph (7)) of seconddisplay panel 200, and gamma transmittance (graph (8)) (gammacharacteristic) corresponding to the gamma value (γ=2.2) for thegradation (the input gradation) corresponding to input video signalData. FIG. 9B is an enlarged view of a low-gradation region in FIG. 9A.Graph (6) (the first transmittance) in FIGS. 9A and 9B representstransmittance in a case where the image is displayed based on thepre-correction gradation (graph (2) in FIGS. 7A and 7B) of first displaypanel 100. As illustrated in FIG. 9B, the first transmittance (graph(6)) is larger than the second transmittance (graph (7)) in the region(the low-gradation region of 0-step gradation to t2-step gradation)lower than or equal to the predetermined input gradation (t2-stepgradation), and the second transmittance (graph (7)) is larger than thefirst transmittance (graph (6)) in the region higher than thepredetermined input gradation (t2-step gradation). When attention ispaid to the transmittance characteristic in FIGS. 9A and 9B, the graphrepresenting the first transmittance of first display panel 100 had afirst inflection point (an upwardly convex portion) in the region lowerthan or equal to the predetermined input gradation (t2-step gradation),and has a second inflection point (a downwardly convex portion) in theregion higher than the predetermined input gradation (t2-stepgradation). That is, the graph (graph (6)) representing the firsttransmittance rises steeply in the region lower than or equal to thepredetermined input gradation (t2-step gradation), and then changesgently. The graph representing the second transmittance of seconddisplay panel 200 has a third inflection point (a downwardly convexportion) in the region lower than or equal to the predetermined inputgradation (t2-step gradation), and has a fourth inflection point (anupwardly convex portion) in the region higher than the predeterminedinput gradation. That is, the graph (graph (7)) representing the secondtransmittance changes gently in the region lower than or equal to thepredetermined input gradation (t2-step gradation), and then risessteeply. The graph representing the first transmittance of first displaypanel 100 and the graph representing the second transmittance of seconddisplay panel 200 change so as to intersect each other at thepredetermined input gradation (t2-step gradation).

In FIG. 10, graph (9), which represents correction transmittance in acase where the image is displayed based on the first gradation (graph(3) in FIGS. 7A and 7B) of first display panel 100, is added to FIG. 9B.Similarly to the gradation characteristic in FIG. 7B, the correctiontransmittance in graph (9) is lower than the first transmittance ingraph (6)) in the region (the low-gradation region of 0-step gradationto t1-step gradation) lower than or equal to predetermined inputgradation (t1-step gradation). The correction transmittance in graph (9)is equal to the first transmittance in the region higher than thepredetermined input gradation (the t1-step gradation).

FIG. 11 is a graph representing a compound transmittance in whichtransmittance (the first transmittance, the correction transmittance) offirst display panel 100 in FIG. 10 and the second transmittance of thesecond display panel 200 are compounded. In FIG. 11, the compoundtransmittance is expressed by a double logarithmic chart. In FIG. 11,graph (11) represents estimated transmittance of first display panel100, and graph (12) represents estimated transmittance of second displaypanel 200. Graph (13) represents transmittance (pre-correction compoundtransmittance) in which the first transmittance (graph (6) in FIGS. 9Aand 9B) of first display panel 100 and the second transmittance (graph(7) in FIGS. 9A and 9B) of second display panel 200 are compounded.Further, graph (14) represents transmittance (post-correction compoundtransmittance) in which the correction transmittance (graph (9) in FIG.10) of first display panel 100 and the second transmittance (graph (7)in FIG. 10) of second display panel 200 are compounded. Graph (15)represents gamma transmittance (gamma characteristic) corresponding tothe gamma value (γ=2.2).

FIG. 12 is a graph representing a comparative example of the graph inFIG. 11. FIG. 12 illustrates compound transmittance in which thetransmittance of the first display panel and the transmittance of thesecond display panel are compounded in a liquid crystal display deviceof the comparative example. In FIG. 12, graph (21) represents estimatedtransmittance of the first display panel of the comparative example,graph (22) represents estimated transmittance of the second displaypanel of the comparative example, and graph (23) representstransmittance (compound transmittance) in which the transmittance of thefirst display panel and the transmittance of the second display panelare compounded. Graph (24) represents the transmittance (the gammatransmittance) (the gamma characteristic) corresponding to the gammavalue (γ=2.2), and is identical to graph (15) in FIG. 11. FIG. 13 is agraph representing transmittance (graph (31)) of the first displaypanel, transmittance (graph (32)) of the second display panel, andtransmittance (graph (33)) (gamma characteristic) corresponding to thegamma value (γ=2.2) for the gradation (input gradation) corresponding tothe input video signal in the liquid crystal display device of thecomparative example. As illustrated in FIG. 13, in all the steps of thegradation, the transmittance (graph (32) in FIG. 13) of the seconddisplay panel is larger than the transmittance (graph (31) in FIG. 13)of the first display panel.

In the comparative example of FIG. 12, the compound transmittance (graph(23)) is largely separated from the gamma transmittance (graph (24)) inthe low-gradation region. Therefore, the black floating occurs and thusdegrades the display quality. On the other hand, in liquid crystaldisplay device 10 according to the present exemplary embodiment, incomparison with the comparative example (the graph indicated by a dottedline), the compound transmittance (the post-correction compoundtransmittance of graph (14)) can brought close to the gammatransmittance in the low-gradation region as illustrated in FIG. 11. Forexample, the compound transmittance can be approximated to the gammatransmittance (the gamma characteristic) up to “2-step gradation”. Whenthe image is displayed in the low gradation, the gradation allocated tofirst display panel 100 is lower than the gradation allocated to thefirst display panel of the comparative example, and the gradationallocated to second display panel 200 is higher than the gradationallocated to the second display panel of the comparative example.Therefore, the degradation of the display quality can be suppressedbecause the black floating is suppressed in comparison with thecomparative example. In a case where the gradation correction processingis not performed in first image data generator 312, the color image isdisplayed based on the pre-correction gradation. Even in this case, asillustrated in graph (13) of FIG. 11, in comparison with the comparativeexample (the graph indicated by the dotted line), the compoundtransmittance (pre-correction compound transmittance) can be broughtclose to the gamma transmittance (gamma characteristic) in thelow-gradation region as illustrated in FIG. 11.

As described above, in a case where the predetermined gammacharacteristic (for example, γ=2.2) is obtained by the compound of thetwo display panels, liquid crystal display device 10 according to thepresent exemplary embodiment sets the gradation (the second gradation)of second display panel 200 such that the transmittance of first displaypanel 100 is larger than the transmittance of second display panel 200in the region lower than or equal to the predetermined gradation, andsuch that the transmittance of second display panel 200 is larger thanthe transmittance of first display panel 100 in the region higher thanthe predetermined gradation. Based on the correction value (see FIG. 8)corresponding to the set second gradation, the input gradation iscorrected and the first gradation is calculated such that the compoundtransmittance comes close to the predetermined gamma transmittance. Atthis time, it is preferable that the second gradation is set lower inthe predetermined gradation or less. Therefore, because an adjustmentwidth (a range of reduction) of the first gradation calculated by thecorrection can be increased, the compound transmittance is easilybrought close to the gamma characteristic.

Liquid crystal display device 10 according to the present exemplaryembodiment is not limited to the above configuration. For example, inputvideo signal Data may be 8 bits. In this case, the gradation isexpressed by 0 to 255 steps.

In the above, the specific embodiments of the present application havebeen described, but the present application is not limited to theabove-mentioned embodiments, and various modifications may be made asappropriate without departing from the spirit of the presentapplication.

What is claimed is:
 1. A liquid crystal display device, in which a plurality of display panels overlap each other and an image is displayed on each of the plurality of display panels, the liquid crystal display device comprising: a first display panel that displays a color image; a second display panel that displays a monochrome image; and an image processor that generates first image data corresponding to the color image and second image data corresponding to the monochrome image based on an input video signal, wherein the image processor generates the first image data and the second image data such that a graph representing transmittance of the first display panel for input gradation corresponding to the input video signal and a graph representing transmittance of the second display panel for the input gradation corresponding to the input video signal intersect each other at predetermined input gradation, and wherein the transmittance of the first display panel is larger than the transmittance of the second display panel in a region lower than or equal to the predetermined input gradation, and the transmittance of the second display panel is larger than the transmittance of the first display panel in a region higher than the predetermined input gradation.
 2. The liquid crystal display device according to claim 1, wherein the graph representing the transmittance of the first display panel has a first inflection point and changes to an upwardly convex state in a region lower than or equal to the predetermined input gradation, and the graph representing the transmittance of the second display panel has a second inflection point and changes to a downwardly convex state in the region lower than or equal to the predetermined input gradation.
 3. The liquid crystal display device according to claim 2, wherein the graph representing the transmittance of the first display panel has a third inflection point and changes to a downwardly convex state in a region higher than the predetermined input gradation, and the graph representing the transmittance of the second display panel has a fourth inflection point and changes to a upwardly convex state in the region higher than the predetermined input gradation.
 4. The liquid crystal display device according to claim 1, wherein the image processor includes a first image data generator that decides first gradation corresponding to the first image data and a second image data generator that decides second gradation corresponding to the second image data, the second image data generator decides the second gradation lower than the first gradation based on the input gradation corresponding to the input video signal in the region lower than or equal to the predetermined input gradation, and the first image data generator decides the first gradation based on the second gradation decided by the second image data generator.
 5. The liquid crystal display device according to claim 4, further comprising a correction table in which the second gradation is correlated with a correction value to be used for correcting the input gradation, wherein the first image data generator refers to the correction table to obtain the correction value, which is correlated with the second gradation decided by the second image data generator, and corrects the input gradation to calculate the first gradation based on the obtained correction value.
 6. The liquid crystal display device according to claim 5, wherein the correction value is set such that a compound transmittance in which the transmittance of the first display panel and the transmittance of the second display panel are compounded comes close to transmittance corresponding to a predetermined gamma value.
 7. A liquid crystal display device, in which a plurality of display panels overlap each other and an image is displayed on each of the plurality of display panels, the liquid crystal display device comprising: a first display panel that displays a color image; a second display panel that displays a monochrome image; and an image processor that generates first image data corresponding to the color image and second image data corresponding to the monochrome image based on an input video signal, wherein the image processor generates the first image data such that a graph representing transmittance of the first display panel for input gradation corresponding to the input video signal has a first inflection point and changes to an upwardly convex state in a region lower than or equal to predetermined input gradation.
 8. The liquid crystal display device according to claim 7, wherein the graph representing the transmittance of the first display panel has a second inflection point and changes to a downwardly convex state in a region higher than the predetermined input gradation.
 9. The liquid crystal display device according to claim 8, wherein the image processor generates the second image data such that a graph representing transmittance of the second display panel for input gradation corresponding to the input video signal has a third inflection point and changes to a downwardly convex state in the region lower than or equal to the predetermined input gradation.
 10. The liquid crystal display device according to claim 9, wherein the graph representing the transmittance of the second display panel has a fourth inflection point and changes to an upwardly convex state in the region higher than the predetermined input gradation.
 11. A liquid crystal display device, in which a plurality of display panels overlap each other and an image is displayed on each of the plurality of display panels, the liquid crystal display device comprising: a first display panel that displays a color image; a second display panel that displays a monochrome image; and an image processor that generates second first image data corresponding to the color image and second image data corresponding to the monochrome image based on an input video signal, wherein the image processor generates the second image data such that a graph representing transmittance of the second display panel for input gradation corresponding to the input video signal has a first inflection point and changes to a downward convex state in a region lower than or equal to predetermined input gradation.
 12. The liquid crystal display device according to claim 11, wherein the graph representing the transmittance of the second display panel has a second inflection point and changes to an upward convex state in a region higher than the predetermined input gradation.
 13. A liquid crystal display device, in which a plurality of display panels overlap each other and an image is displayed on each of the plurality of display panels, the liquid crystal display device comprising: a first display panel that displays a color image; a second display panel that displays a monochrome image; and an image processor that generates first image data corresponding to the color image and second image data corresponding to the monochrome image based on an input video signal, wherein the image processor generates the first image data and the second image data such that a graph representing transmittance of the first display panel for input gradation corresponding to the input video signal and a graph representing transmittance of the second display panel for the input gradation corresponding to the input video signal intersect each other at predetermined input gradation, wherein the image processor includes a first image data generator that decides first gradation corresponding to the first image data and a second image data generator that decides second gradation corresponding to the second image data, wherein the second image data generator decides the second gradation lower than the first gradation based on the input gradation corresponding to the input video signal in a region lower than or equal to the predetermined input gradation, and wherein the first image data generator decides the first gradation based on the second gradation decided by the second image data generator.
 14. The liquid crystal display device according to claim 13, further comprising a correction table in which the second gradation is correlated with a correction value to be used for correcting the input gradation, wherein the first image data generator refers to the correction table to obtain the correction value, which is correlated with the second gradation decided by the second image data generator, and corrects the input gradation to calculate the first gradation based on the obtained correction value.
 15. The liquid crystal display device according to claim 14, wherein the correction value is set such that a compound transmittance in which the transmittance of the first display panel and the transmittance of the second display panel are compounded comes close to transmittance corresponding to a predetermined gamma value. 